1. Field of the Invention
This invention relates to an oily inkjet ink. This invention particularly relates to an oily inkjet cyan ink, which suppresses accumulation of a solid material in the vicinity of nozzles of an inkjet head, and which suppresses jetting-out failure.
2. Description of the Related Art
Inkjet recording systems are the systems in which an inkjet ink having a high degree of fluidity is jetted out in the form of ink droplets from fine head nozzles, and in which an image is recorded by the ink droplets on a printing medium, such as paper. Recently, the inkjet recording systems have rapidly become popular for the possibility of the printing of an image having a high resolution and good image quality by use of a comparatively cheap apparatus. As the inks for use in the inkjet recording systems, there have been proposed various oily inkjet inks, which contain a water-insoluble solvent and a pigment finely dispersed in the water-insoluble solvent.
For example, in U.S. Pat. No. 7,799,123, the applicant proposed an ink, which contains a non-polar solvent, such as an ester solvent, an alcohol solvent, or a hydrocarbon solvent, and a pigment dispersed in the non-polar solvent. The proposed ink has the advantages in that the ink has excellent on-printer stability, and in that the ink yields a printed surface, which does not adhere to a print surface having been printed with an electrophotographic system.
However, ordinarily, in cases where an oily inkjet ink is used for a long period of time in an inkjet printer, a solid material accumulates in the vicinity of the nozzles of the inkjet head. The solid material having thus accumulated obstructs the normal ink jetting-out operation and often causes image failure to occur. Heretofore, it has been considered that the ink clogging is caused to occur by an increase in ink viscosity due to thickening of the ink in the nozzles, coarse particles floating in the ink due to dispersing failure, and pigment agglomerates arising through a change with the passage of time due to bad dispersing stability of pigments. (Reference may be made to, for example, U.S. Pat. No. 7,060,125.)
The ink clogging described above induces jetting-out defects, such as flight bending and scattering of ink droplets, and is therefore undesirable. Therefore, heretofore, in accordance with the aforesaid causes, studies have been conducted on means for suppression of the increase in viscosity by selection of a solvent, a resin, or the like, separation and removal of agglomerated particles by control of a mean particle diameter of the pigment, or suppression of pigment agglomeration by adsorption of a specific dispersant to the pigment.
However, the inventors eagerly conducted study and have found that the jetting-out defects, such as the flight bending and the scattering of the ink droplets, markedly occurs, particularly, with a cyan ink, and that the solid material having agglomerated in the vicinity of the nozzles of the head having been introduced with the cyan ink is actually a water-soluble solid material, regardless of the agglomerated solid material being derived from the oily ink.
The water-soluble solid material is derived from a copper phthalocyanine pigment, which is used as a coloring material in the cyan ink. The water-soluble solid material is dissolved and condensed in water, which is contained in a trace quantity in the ink. Since water is apt to evaporate in the vicinity of the nozzle, the water-soluble solid material deposits due to the evaporation of water and accumulates in the vicinity of the nozzle.
Particularly, in the cases of an oily inkjet ink containing organic solvents having hydrophilic groups, such as an ester solvent and an alcohol solvent, there is a tendency for the water quantity in the ink to increase due to moisture absorption, and the like. Therefore, the water-soluble solid material is apt to be extracted by water in the ink. It has thus been found that the oily inkjet ink containing the organic solvents having the hydrophilic groups, such as the ester solvent and the alcohol solvent, is the ink susceptible to the accumulation of the solid material in the vicinity of the nozzles.